This application claims the priority of German Patent Document No. 100 58 326.1, filed Nov. 24, 2000, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to an inductively coupled high-frequency electron source having a plasma chamber, which is open at least at a first end, and having a gas inlet for a gas to be ionized, as well as a high-frequency coil.
An inductively coupled high-frequency electron source extracts free electrons from a plasma which is maintained by means of an electric alternating field. This field is generated by an induction coil through which a high-frequency current flows. The electrons present in the plasma are accelerated by the electric alternating field to speeds which, in the event of an impact with a neutral atom in the plasma, can cause the ionization of the latter. During the ionization, one or several additional electrons are freed from the neutral atom, whereby a continuing supply of electrons takes place. After some time, the formed ions impact on the wall of the vessel in which the plasma is held on objects which are immersed in the plasma. If the surface of the wall or of the object at the point of the impact is electrically connected with the current source, the ion can again take up the lost negative charge there, whereby the charge compensation is ensured. As a result, the free electrons in the plasma can partially be withdrawn from it, for example, through an opening in the plasma vessel.
Such an electron source is described, for example, in U.S. Pat. No. 5,198,718 in the form of a neutralizer for an ion source which is formed by a plasma chamber with walls made of a dielectric material which is surrounded by a high-frequency coil.
A large portion of the power required for maintaining the plasma in the vessel of an inductively coupled high-frequency electron source is lost because of the fact that high-energy electrons from the plasma impact on the vessel wall and in the process are again bound to atoms. In this process, they also release a large portion of their energy which they had obtained by means of the electric alternating field.
It is therefore an object of the present invention to provide an electron source which has a reduced power requirement.
According to the invention, the interior wall of the plasma chamber is designed to be conductive and electrically connected with a current source. This may affect either the whole surface of the interior wall or only a portion thereof. In addition, the cross-section of openings in the plasma vessel is appropriately dimensioned for the extraction of the electrons. As a result, an effective electrostatic inclusion of the electrons is achieved in the plasma.
Thus, according to the invention, an inductively coupled high-frequency electron source is provided which has a plasma chamber open at least at a first end, the total surface of the open regions of the plasma chamber amounting to a surface amount A0, as well as having a gas inlet for a gas to be ionized and having a high-frequency coil. This high-frequency coil may be arranged and constructed in different fashions; it must only be suitable for sufficiently exciting the plasma. The high-frequency coil may, for example, be arranged coaxially to the longitudinal dimension of the plasma chamber or, in the form of a spiral, adjoin a wall of the plasma chamber, or may be arranged as a cylinder coil adjacent to the plasma chamber. In this case, the high-frequency coil may be arranged either inside or outside the plasma chamber. The interior wall of the plasma chamber is at least partially formed by conductive regions which are connected with a current source. The conductive regions can be formed in different manners, for example, by a conductive coating of a plasma chamber basic body, by inserting conductive bodies, such as plates or a bush or the like; or the plasma chamber itself may consistxe2x80x94at least partiallyxe2x80x94of a conductive material. The total surface of the conductive regions amounts to Ac and the ratio of the surface amounts A0 and Ac is maximally             A      0              A      c        =                              2          ⁢          π          ⁢                      xe2x80x83                    ⁢                      m            e                                    em          0                      ⁢          xe2x80x83        ⁢    with  
with
In a preferred embodiment, it is provided that the conductive regions are interrupted at least once such that currents are prevented in the conductive regions perpendicular to the direction of the magnetic field of the high-frequency coil which penetrates the plasma chamber. As a result, it can be prevented that the high-frequency field induces currents in the conductive regions which could finally act against the excitation effect of the high-frequency field.
In particular, it may be provided that the conductive regions are interrupted a least once in the circumferential direction of the plasma chamber. Such a measure can be used specifically when the high-frequency field penetrates the plasma chamber essentially perpendicularly to the circumferential direction. This means that, no matter at which point the conductive regions are viewed, at least one interruption is always found in the circumferential direction of the plasma chamber. This interruption may, for example, be formed by an inserted nonconductor or a conductive coating may be constructed such that it does not coat the entire interior wall of the plasma chamber but is interrupted in the circumferential direction. This providing of an interruption in the circumferential direction offers an advantage particularly when the high-frequency coil is arranged outside the conductive regions. In this case, the interruption prevents currents from being induced in the circumferential direction in the conductive regions which would hinder the high-frequency field from penetrating into the plasma chamber. The geometrical shape of the interruption can be selected relatively freely if it meets the abovementioned requirements. In particular, it may also be selected such, for example, by means of an appropriately large width of the interruption, that a capacitive electrical field generated by the high-frequency coil is not shielded by the conductive regions. Such a capacitive electrical field can be utilized, for example, for igniting the plasma in plasma chamber.
It may specifically be provided that the conductive regions of the plasma chamber form a conductive bush which, in the circumferential direction of the plasma chamber, has at least one interruption extending essentially in the direction of the longitudinal dimension of the plasma chamber. The bush may therefore, for example, be slotted essentially in the direction of the longitudinal dimension of the plasma chamber, in which case, the direction of the slot may well deviate, for example, by up to 45xc2x0 from the direction of the longitudinal dimension.
As a further development of the invention, the conductive regions may have a smaller dimension than the high-frequency coil at least in the region of one of the ends of the plasma chamber in the direction of the longitudinal dimension. Thus, in these regions, the high-frequency coil overlaps the conductive regions and thus extends closer to at least one of the ends of the plasma chamber than the conductive regions. As a result of such a measure, as an alternative or an addition to the above-mentioned measure of a sufficiently wide interruption, it can also be achieved that the capacitive electrical field of the high-frequency coil can sufficiently penetrate into the plasma chamber and its shielding is therefore prevented.
Basically any suitable material which meets the requirements of such an electron source and its special field of application can be selected for the conductive regions. In particular, the conductive regions may consist of one of the metallic materials titanium, steel, especially stainless steel, for example, austenitically, or may consist of aluminum or tantalum. However, as an alternative, the conductive regions may also consist of a semiconductor, particularly of silicon or germanium.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.